Nuclear reactors, such as boiling water reactors (BWR), have a core that contains fuel assemblies that enclose fuel rods. As the nuclear reactor is a closed circulation system, debris tends to accumulate within the system. Debris in this context refers to any solid material entrained in the fluid flow. Debris can include materials left over from reactor construction, corrosion byproducts, and detritus introduced or dislodged during outages and repair services.
Accumulation of debris within a fuel assembly is potentially damaging to fuel rods. For example, as debris particles lodge against the fuel rods, coolant traveling through the fuel bundle creates turbulence which causes the captured debris particles to vibrate rapidly against the fuel rod cladding, resulting in its perforation or rupture. Fuel rods with damaged cladding are sometimes referred to as “leakers.” If a sufficient number of leakers are present, the plant may be forced to shut down or to operate at less than optimum efficiency in compliance with regulations and to address safety concerns. In either case, it is highly desirable to minimize the entrance of foreign debris into the fuel assemblies.
To prevent debris from entering a fuel assembly, coolant that flows through the fuel assembly is typically filtered at the lower tie plate of the fuel assembly. In this fashion, debris can be prevented from entering into the fuel assembly depending on the effectiveness of the selected filter. This impeded debris simply accumulates within the lower tie plate, but only so long as there is sufficient forward coolant flow through the fuel assembly. The debris that has accumulated within the cavity of the lower tie plate during reactor operations becomes dislodged due to reverse or stagnant coolant flow conditions, or in reaction to an internal or external vibration source. Escape is very likely to occur, for example, as a fuel assembly is moved above the reactor core, which is common during reactor shut down and refueling. As the fuel assembly is lifted upward, the speed of its ascent causes coolant to flow back through the fuel assembly, thereby dislodging (i.e., backwashing) debris that had accumulated in the filter. Once dislodged, the debris falls out of the lower tie plate of the fuel assembly that is being transported and into the lower plenum, or worse, into the vulnerable upper ends of fuel assemblies that are positioned below, in the reactor core.
Debris that has been filtered and has accumulated within the lower tie plate cavity during reactor operations can also backwash out of the lower tie plate upon significant reduction of flow, for example, where reactor flow is reduced at low or no power. This debris will be reintroduced to the lower tie plate in subsequent reactor operation, allowing for a subsequent opportunity for infiltration of the debris into the fuel assembly.
Accordingly, although highly effective filters have been developed, the problem of retaining the debris that has been impeded by the filters remains. Previous attempts to address the debris retention problem employ various filtering structure designs that attempt to prevent debris from being dislodged from the lower tie plate. However, to do so the prior designs substantially change the direction and the momentum of the normal flow pattern of coolant through the lower tie plate, which creates an undesirable pressure drop within the fuel assembly, which adversely affects reactor operation. Further, prior attempts to redesign the structure of conventional lower tie plates resulted in devices that are costly and complicated to manufacture.